1. Field of the Invention
The present invention relates to an apparatus for and a method of analyzing carbon isotopes to determine a carbon isotopic abundance ratio by applying light to a specimen of carbon dioxide (CO.sub.2) and detecting the light absorption spectrum of the specimen.
2. Description of the Related Art
Carbon atoms include isotopes whose mass numbers are 11(.sup.11 C), 12(.sup.12 C), 13(.sup.13 C), and 14(.sup.14 C). The carbon isotopes .sup.12 C, .sup.13 C are stable isotopes which are safe as they cause no radiation exposure unlike the radioactive carbon isotopes .sup.11 C, .sup.14 C. Therefore, research efforts are being made to utilize the carbon isotopes .sup.12 C, .sup.13 C especially Ln the medical field.
Recently, much attention has been attracted among internists to the relationship between an intragastric infection caused by helicobacter pylori (HP) and a gastric ulcer. The .sup.13 C-urea breath test (UBT) has increasingly been used for the diagnosis of an infection caused by HP.
UBT needs to be accompanied by a stable carbon isotope analyzer. Since UBT is conducted clinically, the stable carbon isotope ratio analyzer should preferably be small in size, easily operable for a simple and quick analysis, highly reliable, and inexpensive.
Heretofore, a mass spectrometer (MS) and an infrared spectrometer (IR) are known for use as the stable carbon isotope analyzer. The MS is capable of producing highly accurate outputs, but is difficult to operate and highly expensive. The IR is simple and inexpensive, but low in output accuracy.
There have been proposed analyzers based on laser spectroscopy as new carbon isotope analyzers for solving the above problems. One such analyzer based on laser spectroscopy employs a semiconductor laser which produces a laser emission having a wavelength in a 1.6 .mu.m band. See "Isotope analyzer based on laser spectroscopy and its application to .sup.13 C expiration test (II)", .sup.13 C Medicine, VOL. 4, pp. 8-9, 1994. This analyzer uses a laser diode (LD) of the DFB (distributed feedback) type which is made of an InGaAsP material belonging to the III-V group of the periodic table. The laser diode is capable of single-mode oscillation at normal temperature and is small in size and highly reliable. The laser diode has many practical advantages including easy wavelength sweeping through the control of the laser diode temperature and drive current. Use of the laser diode allows a simple and quick analysis.
However, since the light absorption spectrum of CO.sub.2 has a small absorption intensity in the 1.6 .mu.m band, it is necessary that CO.sub.2 contained in a human exhalation (the concentration of CO.sub.2 in a human exhalation is about 2 to 5%) be concentrated by a pretreatment device before being analyzed in order to increase the amount of light absorption by CO.sub.2. Specifically, the pretreatment device (concentrates CO.sub.2 by solidifying CO.sub.2 contained in a human exhalation with liquid nitrogen, i.e., converting it into dry ice, and thereafter removing unwanted gases of N.sub.2, O.sub.2, etc. with a vacuum pump. Because of the pretreatment device needed to concentrate CO.sub.2, the analyzer is necessarily large in size and expensive to manufacture. The running cost of the analyzer is also high as it requires liquid nitrogen to concentrate CO.sub.2. Another problem is that the analyzer is limited to certain places in its usage as liquid nitrogen is available not at any arbitrary spots.
Since it is known that there exists a spectrum having a large light absorption intensity of CO.sub.2 in the vicinity of wavelengths of 2.0 .mu.m and 4.3 .mu.m (it is necessary to select a spectrum having a large light absorption intensity of .sup.13 CO.sub.2 because a naturally occurring ratio of C for an isotopic analysis is .sup.13 C/.sup.12 C.apprxeq.1/99), the above shortcomings may be overcome by using this spectrum. However, the laser diode of InGaAsP belonging to the III-V group cannot be used in the above wavelength range because it does not continuously oscillate at room temperature in that wavelength range (see Table 1.2, Double-heterojunction crystal and laser oscillation wavelengths", Semiconductor laser and applied techniques, Kougakusha, P48).
A laser diode made of a lead salt material belonging to the IV--IV group oscillates in a frequency range from 4 to 30 .mu.m, but does not continuously oscillate at normal temperature. Therefore, this laser diode needs a large-size freezer which uses liquid helium, liquid nitrogen, or the like for its continuous oscillation. The laser diode also needs to kept in a very low temperature of 4 K or 77 K by an expensive cryostat having a vacuum heat insulation structure. Accordingly, the entire apparatus is large in size, cannot be handled with ease, is highly expensive, and hence does not lend itself to usage at clinical locations.
To solve the above problems, there has been proposed a carbon isotope analyzer which has a spectral laser source comprising an LD-pumped solid-state laser that oscillates in a single mode at normal temperature in a 2 .mu.m band. FIG. 6 of the accompanying drawings shows the proposed carbon isotope analyzer. As shown in FIG. 6, the carbon isotope analyzer has an LD-pumped Tm:YAG solid-state laser 1, a specimen cell 2, a photodetector 3, a lock-in amplifier 4, a piezoelectric device controller 5 for controlling a piezoelectric device 12 (see FIG. 7 of the accompanying drawings), and an oscillator 6 for frequency-modulating oscillated light of the solid-state laser 1.
The solid-state laser 1 is schematically shown in FIG. 7. As shown in FIG. 7, the solid-state laser 1 has a high-output laser diode 7, an optical system 8 which optically couples emitted light from the laser diode 7 to a laser resonator, and a Tm:YAG rod 9 having an end "a" facing the optical system 8 and an opposite end "b" facing a wavelength selector 10. The end "a" is coated with a layer which has a high transmittance with respect to exciting wavelengths and a high reflectance with respect to oscillated wavelengths, and the end "b" is coated with a layer which has a low reflectance with respect to oscillated wavelengths. The wavelength selector 10 comprises a combination of birefringent filters, etalon, or the like. The solid-state laser 1 also includes an output mirror 11 for the laser resonator, and the piezoelectric device 12 which is mounted on the output mirror 11.
The end "a" of the Tm:YAG rod 9 and the output mirror 11 jointly make up the laser resonator. Exciting light emitted from the high-output laser diode 7 is coupled by the optical system 8 to the Tm:YAG rod 9 to match an oscillation mode of the laser resonator. As a result, the Tm:YAG rod 9 is excited with high efficiency, causing the laser resonator to oscillate a laser beam which is outputted from the output mirror 11. The wavelength of the oscillated laser beam is selected to be any arbitrary wavelength in the vicinity of 2 .mu.m by the wavelength selector 10 which is inserted in the laser resonator.
When a bias voltage and a modulation voltage are applied to the piezoelectric device 12 mounted on the output mirror 11, the length of the laser resonator is slightly varied to modulate the frequency of the oscillated laser beam.
The solid-state laser 1 is controlled by the wavelength selector 10 to cause its oscillated wavelength to sweep a range which covers the spectrums of both .sup.13 CO.sub.2 and .sup.12 CO.sub.2. A modulation signal from the oscillator 6 is amplified by the piezoelectric device controller 5, and applied, together with the bias voltage, to the piezoelectric device 12 of the solid-state laser 1. In this manner, the oscillation spectrum of the solid-state laser 1 is modulated.
The output laser light from the solid-state laser 1 which is thus frequency-modulated and wavelength-swept is introduced into the specimen cell 22. The specimen cell 2 contains a specimen, i.e., a CO.sub.2 gas, which has been introduced from a specimen gas inlet 2a under a pressure sufficient enough to distinguish the fine structure of the light absorption spectrum of CO.sub.2. The laser light which has entered the specimen cell 2 acts with the specimen, i.e., CO.sub.2, in the specimen cell 2, causing a resonance absorption. The light which leaves the specimen cell 2 is detected by the photodetector 3, which applies an output signal to the lock-in amplifier 4 that detects only a signal synchronized with the frequency of the oscillator 6. As a result of such synchronous detection, the lock-in amplifier 4 can detect a signal of high S/N while removing fluctuations of the light output due to wavelength sweeping and noise inherent in the solid-state laser. The specimen gas is discharged from the specimen cell 2 from a specimen gas outlet 2b.
The signal from the photodetector 3 is detected by the lock-in amplifier 4 as a second derivative (2f spectrum) of the form of the light absorption spectrum (see FIG. 8 of the accompanying drawings). Peak values of the obtained light absorption spectrum signals of .sup.12 CO.sub.2 and .sup.13 CO.sub.2 are determined, and a ratio of the absorption intensities of .sup.12 CO.sub.2 and .sup.13 CO.sub.2 is determined from the peak values as an isotopic abundance ratio.
In the carbon isotope analyzer shown in FIG. 6, the spectrum in the vicinity of 2 .mu.m where the light absorption intensity of CO.sub.2 is high is measured through the lock-in amplifier 4, and the laser light is utilized substantially in its entirety. Therefore, the isotopic abundance ratio can be measured with high sensitivity. As a consequence, the pretreatment device, referred to above, may be dispensed with.
However, the carbon isotope analyzer shown in FIG. 6 is disadvantageous for the following reasons: The wavelength sweeping is carried out by controlling the wavelength selector 10 to produce the spectrum shown in FIG. 8 (with about 2000 samples in the direction of the wavelength sweeping). Since the wavelength selector 10 needs to be mechanically varied with ultrahigh precision for the wavelength sweeping, the wavelength sweeping is time-consuming and the time required for the analysis is quite long. If the time interval between the measurements of the spectrums of .sup.12 CO.sub.2 and .sup.13 CO.sub.2 is long, then the pressure of the specimen gas in the specimen cell 2 may vary due to a gas adsorption and a temperature change during that interval. This results in the measurement of a spectrum under a different pressure, and hence a reduced level of measurement accuracy. Furthermore, inasmuch as the wavelergth selector 10 is controlled with ultrahigh precision, it is susceptible to disturbances including oscillations, heat changes, etc., and low in reliability.